Fall is a good time to begin learning to fly in most parts of the United States because you can take advantage of the lull between the thunderstorms of spring and summer, and winter's large storms. Another advantage is that the atmosphere is generally more stable than during the warmer seasons. This stability calms the up-and-down air motions that cause turbulence, which can be disconcerting to a new pilot.
A disadvantage of fall flying is that calmer weather combined with shorter days can produce fog. Fog isn't exciting--you'll never see videos of adventurous weather fans chasing fog the way they chase tornadoes--but it is dangerous to those who fly.
History's worst aviation accident happened in the fog that covered the airport at Tenerife in the Canary Islands on March 27, 1977, when a KLM Boeing 747 started its takeoff run without a clearance although the captain thought he had been cleared to take off. That 747 smashed into a Pan Am Boeing 747 taxiing on the runway. The fog hid the Pam Am 747 until the KLM airplane was too close to avoid it. The crash killed 570 people on the two airplanes; 56 passengers and five crewmembers, all from the Pan Am flight, survived.
The possibility of a runway collision is only one of fog's dangers. For a general aviation pilot a more likely danger is flying to an airport only to find fog hiding the runway. If the fog hasn't reduced visibility below the required minimum, an instrument-rated pilot in an airplane equipped for an instrument landing should be able to land safely. Otherwise, the only safe option is to fly to another airport with good visibility, if the airplane has enough fuel.
Imagine flying over an airport that is covered by a 300-foot-deep fog layer. After looking down to clearly see the lights outlining the runway and taxiway, you enter the pattern and things still look good. But, as you descend on final approach, the lights grow dimmer until you see nothing but white fog in the glare of your landing light.
What happened? When you looked down from above the airport, you were looking through about 300 feet of fog. But, on final as you reached the top of the fog--assuming a normal 3-degree glidepath on your approach--around 5,700 feet of fog was between you and the runway, with millions of tiny water drops blocking the view.
The way to avoid dangerous encounters with fog is to obtain a complete preflight briefing, and know enough about fog to understand the briefing.
Fog is nothing more than a cloud that's on the ground. The National Weather Service defines it as "A visible aggregate of minute water particles that are based at the Earth's surface, which reduces horizontal visibility to less than five-eighths of a statute mile." If the visibility is less than a quarter-mile, it's dense fog. A thin fog is called mist, which the Weather Service defines as "A visible aggregate of minute water particles suspended in the atmosphere that reduces visibility to less than seven statute miles but greater than or equal to five-eighths of a statute mile." In METARs (reports of existing weather conditions) and terminal aerodrome forecasts (the expected weather) the weather service abbreviates mist as BR and fog as FG. If dense fog is expected, the forecast visibility will be given as less than a quarter of a mile.
Forecasters base predictions of fog on when the air will cool to a temperature called the dew point. The amount of invisible water vapor that can be in the air depends on the temperature. The higher the temperature, the more water vapor can stay in the air. As the amount of water vapor in the air increases, the dew point goes up.
Regular weather observations include measurements of water vapor in the air. The air's humidity is reported as the dew point, which is the temperature at which the water vapor in the air will begin to condense. If an afternoon observation reports a temperature of 70 degrees F and a dew point of 55 degrees, dew will begin forming on the grass and airplanes tied down on the ramp when the temperature cools to 55 degrees. If conditions are right, fog will begin forming right above the ground when the temperature cools to the dew point temperature.
Dew can start forming before the official temperature drops to the dew point because thermometers used for official reports are approximately five feet above the ground and the air right at the ground below them could be a degree or two colder. Dew should alert you to the possibility that as the temperature drops a little more, fog could begin forming.
If you're planning to go for an evening flight you need to pay attention to the observed dew points at airports where you might want to land. If they are within five or so degrees of the temperature, fog is likely as the ground begins cooling after sunset. Fog is likely to form sooner at airports in valleys because as air cools, it becomes denser and flows downhill into valleys.
If you are planning an early morning departure you should also be aware of the possibility of fog that could delay your takeoff or cover your destination airport. A day's coldest temperatures usually occur soon after sunrise, which is why you might see only a little fog in low places along the road while driving to the airport in the dark, only to have more widespread fog at the airport. You will have to wait for the sun s heat to evaporate the fog, which usually doesn't take too long.
Radiation fog is the most common kind, but anything that cools the air to its dew point can cause fog to form. Relatively warm, humid air flowing over cold surfaces, especially snow-covered ground or cold water, can be cooled to the dew point to form advection fog. Meteorologists use advection to refer to the horizontal transport of atmospheric properties such as heat and humidity. Unlike ground fog, advection fog can cover wide areas of the country, such as Midwestern states in the wake of a winter snowstorm. As the storm moves away winds could shift to blow from the south and bring in humid Gulf of Mexico air.
Unlike ground fog, advection fog can arrive at any time of the day and persist for hours, closing airports over a wide area.
Since rising air cools, air flowing uphill with the wind can cool to the dew point, forming upslope fog. Widespread upslope fog occurs when humid air flows over land that rises slowly, such as on the Piedmont east of the Appalachian Mountains in the East and over the Great Plains in the West.
Most fog forms when the air is cooled to the dew point, but some kinds form when water vapor is added to the air. Evaporation from falling rain or drizzle can do this. When evaporation increases the dew point to match the temperature, the water vapor condenses to form precipitation fog.
Fog is difficult to forecast because detailed information about the amount of water vapor in the air often is not available. Improved fog forecasts are on the way--in December 2004 Troposphere Airborne Meteorological Data Reporting (TAMDAR) sensors were installed on several commuter airliners flying in the Midwest. This system collects weather data, including humidity, automatically as the airplanes climb, cruise, and descend; the data is radioed to forecasters (see "The Weather Never Sleeps: Piecing Together Weather," June 2005 AOPA Flight Training). Meteorologists using the system are reporting improved forecasts for fog, icing, and other kinds of dangerous weather. The system now covers the contiguous 48 states and Alaska, and sensors are being added to more aircraft. No matter how much additional data helps forecasters improve their predictions of when fog will form and dissipate, pilots remain responsible for using their knowledge and the available information to ensure safe flights.
Jack Williams, a science writer specializing in weather and climate, is an instrument-rated private pilot. The latest of his six books is The AMS Weather Book: The Ultimate Guide to America's Weather. He answers questions on his Web site.
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